1. Field of the Invention
The present invention relates to an image capture apparatus and a control method therefor, and in particular to an image capture apparatus with an anti-shake function and a control method therefor.
2. Description of the Related Art
Conventionally, a so-called image stabilization function is known that corrects (alleviates) a shake of a captured image (an image shake) caused by a movement of an image capture apparatus, and representative methods for realizing the image stabilization function include optical image stabilization and electronic image stabilization (Japanese Patent Laid-Open No. 2011-145604). The optical image stabilization is a method for reducing a movement of an image of a subject by moving optical elements such as a shift lens and an image sensor along a plane perpendicular to an optical axis based on a detected amount of shake of an image capture apparatus. On the other hand, the electronic image stabilization is a method for reducing a movement of an image of a subject by setting an effective pixel region smaller than a capturable range and shifting the position of the effective pixel region based on a detected amount of shake of an image capture apparatus. They are both techniques to reduce a shake of a captured image (an image shake) caused by a movement of an image of a subject in vertical and horizontal directions (translation of an image).
Such an optical anti-shake function is based on a movement of an image capture apparatus obtained using sensors that detect a rotational motion around a yaw axis (horizontal axis) (in a left-right direction), as well as a rotational motion around a pitch axis (vertical axis) (in an up-down direction). These sensors, however, cannot detect a translational motion in three directions (a motion parallel to coordinate axes) and a rotational motion around a roll axis (optical axis) among six-degree-of-freedom motions of the image capture apparatus. While the translational motion can be detected by providing an acceleration sensor as described in Japanese Patent Laid-Open No. 2010-25961, this is not easy for implementation and cost reasons. The same goes for a sensor that detects a rotational motion around the roll axis.
Meanwhile, Japanese Patent Laid-Open No. 2008-259076 proposes a technique to correct an image shake caused by a six-degree-of-freedom movement of an image capture apparatus (a rotational motion in three directions and a translational motion in three directions) by applying geometric deformation processing to a captured image in accordance with a motion vector detected from the captured image. An image correction technique using the geometric deformation processing can be utilized not only for correction of an image shake caused by a movement of the image capture apparatus, but also for correction of optical aberration, correction of a rolling shutter distortion unique to a CMOS image sensor, correction of a distortion that occurs in a case where an image of a subject is captured from below (projection distortion), etc. In view of this, it is thought that more advanced anti-shake effects can be achieved by applying the geometric deformation processing to an image for which optical hand movement correction has been performed (optical anti-shake image).
However, a method for detecting and estimating an image distortion using a motion vector at the time of application of the geometric deformation processing lowers the accuracy depending on scenes, increases an amount of computation necessary for high-accuracy estimation, and makes the estimation itself difficult. For example, in the case of a low-contrast image, such as an image captured indoors, it is more likely that a motion vector fail to be detected and an erroneous motion vector is detected. Therefore, for example, if a rolling shutter distortion is corrected in accordance with a moving subject, there is a possibility of the occurrence of harmful effects, e.g., a distortion of a portion that is supposed to be still, and a continuous image shake after the correction.
Furthermore, if a rolling shutter distortion in a roll direction is estimated from a motion vector simultaneously with the shake of the image capture apparatus, there will be more estimation variables, leading to an explosive increase in an amount of computation and destabilization of solution estimation. Furthermore, it is basically not easy to estimate a rolling shutter distortion from a motion vector with high accuracy. Moreover, the difficulty will further increase if correction parameters for a radial distortion and optical aberration such as transverse chromatic aberration are simultaneously estimated from a motion vector.